U.S. patent application number 13/185798 was filed with the patent office on 2012-01-26 for electrode material and solid oxide fuel cell containing the electrode material.
This patent application is currently assigned to NGK Insulators, Ltd.. Invention is credited to Shinji Fujisaki, Ayano KOBAYASHI, Makoto Ohmori.
Application Number | 20120021334 13/185798 |
Document ID | / |
Family ID | 45493899 |
Filed Date | 2012-01-26 |
United States Patent
Application |
20120021334 |
Kind Code |
A1 |
KOBAYASHI; Ayano ; et
al. |
January 26, 2012 |
ELECTRODE MATERIAL AND SOLID OXIDE FUEL CELL CONTAINING THE
ELECTRODE MATERIAL
Abstract
The electrode material contains a complex oxide and at least one
of ZrO.sub.2 and a compound comprising ZrO.sub.2. The complex oxide
has a perovskite structure represented by a general formula
ABO.sub.3. ZrO.sub.2 is contained in an amount of 0.3'10.sup.-2 wt
% to 1 wt % relative to the entire electrode material.
Inventors: |
KOBAYASHI; Ayano;
(Nagoya-City, JP) ; Fujisaki; Shinji;
(Kuwana-City, JP) ; Ohmori; Makoto; (Nagoya-City,
JP) |
Assignee: |
NGK Insulators, Ltd.
Nagoya-City
JP
|
Family ID: |
45493899 |
Appl. No.: |
13/185798 |
Filed: |
July 19, 2011 |
Current U.S.
Class: |
429/489 ;
252/182.1; 252/520.2; 429/527; 429/528 |
Current CPC
Class: |
Y02E 60/50 20130101;
H01M 2008/1293 20130101; H01M 4/8835 20130101; H01M 4/9033
20130101 |
Class at
Publication: |
429/489 ;
429/527; 429/528; 252/182.1; 252/520.2 |
International
Class: |
H01M 8/10 20060101
H01M008/10; H01M 4/90 20060101 H01M004/90; H01M 4/48 20100101
H01M004/48 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 21, 2010 |
JP |
2010-164011 |
May 20, 2011 |
JP |
2011-114049 |
Claims
1. An electrode material comprising: a complex oxide having a
perovskite structure represented by a general formula ABO.sub.3, at
least one of ZrO.sub.2 and a compound comprising ZrO.sub.2, and
ZrO.sub.2 being contained in an amount of 0.3.times.10.sup.-2 wt %
to 1 wt % relative to the entire electrode material.
2. The electrode material according to claim 1, wherein the A site
includes at least one of La and Sr.
3. The electrode material according to claim 1, wherein the complex
oxide has oxygen ion conductivity and electron conductivity.
4. The electrode material according to any of claims 1, wherein the
complex oxide is (LaSr)(CoFe)O.sub.3, (LaSr)FeO.sub.3,
(LaSr)CoO.sub.3, La(NiFe)O.sub.3, or (SmSr)CoO.sub.3.
5. The electrode material according to claim 1, wherein the
electrode material is a powder having an average particle diameter
of 20 .mu.m or less.
6. The electrode material according to claim 5, wherein the powder
has an average particle diameter of 1.0 .mu.m or less.
7. A solid oxide fuel cell comprising: a cathode composed of the
electrode material of claim 1; an anode; and a solid electrolyte
layer disposed between the cathode and the anode.
8. A solid oxide fuel cell comprising: a cathode; an anode; a solid
electrolyte layer disposed between the cathode and the anode, and
the cathode including zirconia, lanthanum zirconate, or strontium
zirconate in a surface region formed along the solid electrolyte
layer.
9. A solid oxide fuel cell comprising: a cathode; an anode; a solid
electrolyte layer disposed between the cathode and the anode, and
the cathode including zirconia, lanthanum zirconate, or strontium
zirconate in a inner region in a thickness direction.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to Japanese Patent
Application No. 2010-164011 filed on July 21, 2010 and Japanese
Patent Application No. 2011-114049, filed on May 20, 2011. The
entire disclosure of Japanese Patent Application No. 2010-164011
and Japanese Patent Application No. 2011-114049 is hereby
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an electrode material and a
solid oxide fuel cell containing the electrode material.
[0004] 2. Description of the Related Art
[0005] In recent years, fuel cells have been attracting attention
from the environmental viewpoint and from the viewpoint of
effective use of energy resources, and several materials and
structures have been proposed for fuel cells.
[0006] Patent Document (see Japanese Patent Application Laid-Open
No. 2006-32132) discloses use of LSCF powder as base powder of the
cathode of a solid oxide fuel cell (SOFC).
SUMMARY OF THE INVENTION
[0007] However, in a fuel cell, peeling may occur at the interface
between the electrode and an other layer that is in contact with
the electrode.
[0008] Such peeling, once it occurs, deteriorates the output
characteristics.
[0009] An object of the present invention is to provide a novel
electrode material that can stably function over a long period of
time by inhibiting peeling at the interface between an electrode
and an other layer, and to provide a solid oxide fuel cell
containing the electrode material.
[0010] The inventors, as a result of having conducted diligent
research to address the above-described problem, found that a
suitable amount of zirconia (ZrO.sub.2) contained in a cathode
inhibits peeling at the interface between the cathode and an other
layer that is in contact with the cathode.
[0011] That is, the electrode material according to the first
aspect of the present invention is provided with a complex oxide
and at least one of ZrO.sub.2 and a compound comprising ZrO.sub.2.
The complex oxide has a perovskite structure represented by a
general formula ABO.sub.3. ZrO.sub.2 is contained in an amount of
0.3.times.10.sup.-2 wt % to 1 wt % relative to the entire electrode
material.
[0012] The solid oxide fuel cell according to the second aspect of
the present invention is provided with a cathode composed of the
electrode material, an anode, and a solid electrolyte layer
disposed between the cathode and the anode.
[0013] For example, when applied to the electrode of a fuel cell,
the electrode material can inhibit generation of peeling at the
interface between the electrode and another component that is
disposed so as to be in contact with the electrode, thereby
enabling the fuel cell to stably function over a long period of
time.
[0014] The electrode material is suitable as, for example, a
material for forming the electrode of a fuel cell. An electrode
formed with the electrode material can inhibit peeling at the
interface between the electrode and a layer that is disposed so as
to be in contact with the electrode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a cross sectional view showing the structure of
the principal part of a fuel cell.
[0016] FIG. 2 is an SEM image showing the microstructure of a
cathode before a thermal cycling test.
[0017] FIG. 3 is an SEM image showing a cathode containing 0.3 wt %
of zirconia after a thermal cycling test.
[0018] FIG. 4 is an SEM image showing a cathode containing 0.001 wt
% of zirconia after a thermal cycling test.
DETAILED DESCRIPTION OF THE INVENTION
[0019] 1. Electrode Material
[0020] The electrode material contains a complex oxide having a
perovskite structure and at least one of zirconia (ZrO.sub.2) and a
compound containing zirconia (for example, yttria-stabilized
zirconia (YSZ) or the like).
[0021] The composition of the complex oxide is represented by a
general formula ABO;. The A site may include at least one of La and
Sr.
[0022] Specific examples of such complex oxides include LSCF or
(LaSr)(CoFe)O.sub.3, LSF or (LaSr)FeO.sub.3, LSC or
(LaSr)CoO.sub.3, LNF or La(NiFe)O.sub.3, SSC or (SmSr)CoO.sub.3,
and like materials. These complex oxides are materials that have
both oxygen ion conductivity and electron conductivity, and are
called mixed conductive materials.
[0023] The electrode material may contain the complex oxide as a
"principal component." The phrase "composition X contains material
Y as a principal component" means that material Y accounts for
preferably 60 wt % or greater, more preferably 70 wt % or greater,
and still more preferably 90 wt % or greater relative to the entire
composition X.
[0024] The electrode material may be a powder. The average particle
diameter of the powder is preferably 20 .mu.m or less, more
preferably 5.0 .mu.m or less, and still more preferably 1.0 .mu.m
or less. The average particle diameter may be measured by a laser
diffraction/scattering particle size distribution analyzer (LA-700,
manufactured by Horiba Ltd.).
[0025] Zirconia is contained in an amount of 0.3.times.10.sup.-2 wt
% to 1 wt % relative to the entire electrode material. The amount
of zirconia can be measured by, for example, subjecting the
electrode material to inductively coupled plasma atomic emission
spectroscopy (ICP-AES).
[0026] The electrode material may contain a component other than
the complex oxide and zirconia.
[0027] 2. Method for Producing Electrode Material
[0028] An example of a method for producing the electrode material
of section 1 above will now be described below.
[0029] Specifically, the production method includes obtaining a
complex oxide having a perovskite structure, triturating the
complex oxide, and adding zirconia to the complex oxide.
[0030] Examples of methods for obtaining a complex oxide having a
perovskite structure include solid phase processes, liquid phase
processes (such as citrate process, Pechini process, and
co-precipitation process), and the like.
[0031] Trituration is performed with, for example, a ball mill. The
complex oxide may be pulverized before trituration. That is, it is
possible that a lump of a material having a perovskite structure is
prepared, broken down (pulverized) so as to have a diameter of 200
.mu.m or less, and triturated more finely. By pulverization and
trituration, the average particle diameter of the material is
controlled to 20 .mu.m or less, 5 .mu.m or less, or 1 .mu.m or
less.
[0032] It is preferable that the additive amount of zirconia is
controlled to 0.3.times.10.sup.-2 wt % or greater relative to the
entire electrode material. The additive amount of zirconia may be
controlled to no more than 1 wt % relative to the entire electrode
material. Addition of zirconia may be performed such that zirconia
powder is mixed with a triturated complex oxide having a perovskite
structure, or zirconia strips are triturated together with a
complex oxide that is not triturated. Moreover, by adding a
zirconia-containing compound (such as yttria-stabilized zirconia
(YSZ)) or by adding both a zirconia-containing compound and
zirconia, the additive amount of zirconia may be controlled so as
to be within the aforementioned range.
[0033] 3. Fuel Cell (Solid Oxide Fuel Cell)
[0034] A solid oxide fuel cell (SOFC) will now be described as an
example of a fuel cell. In particular, an SOFC stack having a cell
stack structure in which a plurality of fuel cells are stacked will
be mainly described below.
[0035] 3-1. Outline of Fuel Cell Stack
[0036] As shown in FIG. 1, a fuel cell stack 10 is provided with a
fuel cell 1 (hereinafter simply referred to as a "cell") and a
current collector 4.
[0037] 3-2. Outline of Cell 1
[0038] The cell 1 is a thin plate of ceramic. The thickness of the
cell 1 is, for example, 30 .mu.m to 700 .mu.m, and the diameter of
the cell 1 is, for example, 5 mm to 50 mm. As shown in FIG. 1, the
cell 1 is provided with an anode 11, a barrier layer 13, a cathode
14, and an electrolyte layer (solid electrolyte layer) 15.
[0039] 3-3. Anode
[0040] As the material of the anode 11, for example, a material
that is for use in forming an anode in a known fuel cell is used.
More specific examples of the material of the anode 11 include
nickel oxid-yttria-stabilized zirconia (NiO--YSZ) and/or nickel
oxide-yttria (NiO--Y.sub.2O.sub.3). The anode 11 can contain these
materials as principal components.
[0041] The anode 11 may function as a substrate that supports other
layers included in the cell 1 (the substrate may also be referred
to as a support). That is, the anode 11 may have the largest
thickness among the layers included in the cell 1. Specifically,
the thickness of the anode 11 may be 10 .mu.m to 600 .mu.m.
[0042] Electric conductivity can be imparted to the anode 11 by
subjecting the anode 11 to a reduction treatment (for example, a
treatment to reduce NiO to Ni).
[0043] Moreover, the anode 11 may have two or more layers. For
example, the anode 11 may have two layers, i.e., a substrate and an
anode active layer (fuel side electrode) formed thereon. Materials
of the substrate and the anode active layer can be selected from
the materials of the anode 11 described above. More specifically, a
substrate composed of NiO--Y.sub.2O.sub.3 and an anode active layer
composed of NiO--YSZ may be combined.
[0044] 3-4. Barrier Layer
[0045] The barrier layer 13 is provided between the cathode 14 and
the anode 11, and more specifically the barrier layer 13 is
provided between the cathode 14 and the electrolyte layer 15.
[0046] The barrier layer 13 contains cerium. The barrier layer may
contain cerium in the form of ceria (cerium oxide). Specific
examples of the material of the barrier layer 13 include ceria and
ceria-based materials containing a rare earth metal oxide and
forming a solid solution with ceria. The barrier layer 13 can
contain a ceria-based material as a principal component.
[0047] Specific examples of the ceria-based material include
gadolinium-doped ceria (GDC: (Ce,Gd)O.sub.2), samarium-doped ceria
(SDC: (Ce,Sm)O.sub.2), and the like. The concentration of rare
earth metal in the ceria-based material is preferably 5 to 20 mol
%. The barrier layer 13 may contain an additive in addition to the
ceria-based material.
[0048] The thickness of the barrier layer 13 may be 30 .mu.m or
less.
[0049] The barrier layer 13 can inhibit diffusion of cation from
the cathode 14 into the electrolyte layer 15. That is, the barrier
layer 13 can inhibit a decrease of output density and extend the
life of the cell 1.
[0050] 3-5. Cathode
[0051] The cathode 14 is composed of the electrode material
described in section 1 above. The thickness of the cathode 14 may
be about 5 .mu.m to 50 .mu.m.
[0052] In the case where the cathode 14 is in contact with an other
layer, the presence of zirconia in a surface region of the cathode
14 formed along the other layer in the thickness direction inhibits
peeling at the interface between the cathode 14 and the other
layer. This appears to be the effect brought about by the formation
of a solid solution between zirconia in the cathode 14 and the
component of the other layer.
[0053] While repetitive power generation may result in degradation
(change in microstructure) of the cathode 14, the presence of
zirconia in the inner region of the cathode 14 in the thickness
direction can strengthen the skeleton inside the cathode 14. It is
thus possible to inhibit a change in the microstructure of the
cathode 14.
[0054] Meanwhile, zirconia and another component present in the
cathode may react. The obtained reaction product may result in a
phenomenon of impaired output density because of an increased
electric resistance value of the cell, decreased reaction active
spots, the non-uniform composition of the cathode, and the like.
However, these phenomena are also inhibited when the amount of
zirconia is 1 wt % or less.
[0055] The above-described effects of adding zirconia are the
inventors' own findings.
[0056] In addition to attaining the effects described above,
zirconia is used also to impart oxygen ion conductivity to an
electron conductive material. For example, since (LaSr)MnO.sub.3,
or LSM, is an electron conductive material that does not have
oxygen ion conductivity, LSM can be used, after being mixed with
zirconia, in the form of a composite material in the case where LSM
is used for a cathode.
[0057] When zirconia is used to attain oxygen ion conductivity, the
ratio of LSM to zirconia mixed is, however, about 1:1. As described
above, the amount of zirconia in the electrode material in this
embodiment is much lower (no more than 1 wt %) than the amount of
zirconia for attaining oxygen ion conductivity. That is, the
inventors have found the specific effect brought about, not by the
addition of a large amount of zirconia as used to impart oxygen ion
conductivity, but by the addition of a small amount of
zirconia.
[0058] While no specific discussion is presented in the above
description, in the case where lanthanum (La) is contained in the
electrode material, at least part of zirconium constituting
zirconia added to the electrode material may be present in the form
of lanthanum zirconate (La.sub.2Zr.sub.2O.sub.7) in the cathode 14.
Similarly, in the case where strontium (Sr) is contained in the
electrode material, at least part of zirconium constituting
zirconia added to the electrode material may be present in the form
of strontium zirconate (SrZrO.sub.3) in the cathode 14.
[0059] In the case where the cathode 14 is in contact with an other
layer, the presence of lanthanum zirconate or strontium zirconate
in a surface region of the cathode 14 formed along the other layer
in the thickness direction inhibits peeling at the interface
between the cathode 14 and the other layer.
[0060] Also, the presence of lanthanum zirconate or strontium
zirconate in the inner region of the cathode 14 in the thickness
direction can strengthen the skeleton inside the cathode 14. It is
thus possible to inhibit a change in the microstructure of the
cathode 14.
[0061] Whether zirconium is in the form of zirconia, lanthanum
zirconate, or strontium zirconate can be determined by, for
example, analyzing a diffraction pattern of a transmission electron
microscope (TEM).
[0062] Although the barrier layer 13 is in contact with the cathode
14 in this embodiment, the solid electrolyte layer 15, for example,
may be in contacted with the cathode 14.
[0063] 3-6. Electrolyte Layer
[0064] The electrolyte layer 15 is provided between the barrier
layer 13 and the anode 11.
[0065] The electrolyte layer 15 contains zirconium. The electrolyte
layer 15 may contain zirconium in the form of zirconia (ZrO.sub.2).
Specifically, the electrolyte layer 15 can contain zirconia as a
principal component. The electrolyte layer 15 can contain, in
addition to zirconia, additives such as Y.sub.2O.sub.3 and/or
Sc.sub.2O.sub.3. Such additives can function as stabilizers. The
amount of additive in the electrolyte layer 15 is about 3 to 20 mol
%. That is, examples of the material of the electrolyte layer 15
include zirconia-based materials such as yttria-stabilized
zirconia, e.g., 3YSZ, 8YSZ, and 10YSZ; scandia-stabilized zirconia
(ScSZ); and the like.
[0066] The thickness of the electrolyte layer 15 may be 30 .mu.m or
less.
[0067] 3-7. Current Collector
[0068] The current collector 4 is provided with a plurality of
conductive connectors 41.
[0069] As shown in FIG. 1, an conductive connector 41 is a
depression provided in the current collector 4, and the bottom
thereof is connected to the cathode 14 via a conductive adhesive
411. The bottom of the conductive connector 41 has a portion that
is discontinuous with its surroundings.
[0070] During power generation, fuel gas is supplied to the anode
11. Air is supplied to the cathode 14 by blowing air toward the
side-surface of the cell stack structure (for example, toward the
surface of the paper showing FIG. 1).
[0071] Although not shown, the fuel cell stack 10 is further
provided with a lead wire that sends the electric current generated
in the cell stack 10 to an external apparatus, a gas reformer that
includes, e.g., a catalyst to reform fuel gas, and a like
member.
[0072] 4. Method for Producing Fuel Cell
[0073] 4-1. Formation of Anode
[0074] The anode 11 can be formed by compacting molding. That is,
the formation of the anode 11 may include introducing mixed powder
of the materials of the anode 11 into a mold and compacting the
powder to give a green compact.
[0075] The materials of the anode 11 are as discussed in connection
with the configuration of the fuel cell in the description provided
above. For example, nickel oxide, zirconia, and optionally a
pore-forming agent are used as the materials. The pore-forming
agent is an additive to create holes in the anode. As the
pore-forming agent, a material that disappears in a subsequent
process is used. An example of such a material may be cellulose
powder.
[0076] The ratio of the materials mixed is not particularly limited
and is suitably set according to the properties required of the
fuel cell.
[0077] Also, the pressure applied to the powder during compacting
molding is set such that the anode has sufficient rigidity.
[0078] The internal structure of the anode 11, e.g., a gas passage
(not shown), may be formed by performing compacting molding with a
member that is eliminated when calcined (a cellulose sheet or the
like) being arranged inside the powder, and then performing
calcination.
[0079] 4-2. Formation of Electrolyte Layer
[0080] The method for producing a fuel cell includes forming an
electrolyte layer on the green body of the anode formed by
compacting molding.
[0081] Examples of methods for forming an electrolyte include cold
isostatic pressing (CIP) method and thermocompression bonding both
of which use an electrolyte material processed into a sheet form,
and slurry dip method in which an anode is dipped into an
electrolyte material that has been prepared so as to take a slurry
form. In CIP method, the pressure applied during the compression
bonding of the sheet is preferably 50 to 300 MPa.
[0082] 4-3. Calcination
[0083] The method for producing a fuel cell includes co-calcining
(co-sintering) the anode that has been compacting-molded and the
electrolyte layer. Conditions such as calcination temperature and
calcination time are set according to the materials of the cell and
other factors. The calcination temperature can be set to, for
example, about 1350.degree. C. to 1500.degree. C., and the
calcination time can be set to, for example, about 1 hour to 20
hours.
[0084] 4-4. Degreasing
[0085] Degreasing may be performed before the calcination described
in section 4-3 above. Degreasing is performed by heating.
Conditions such as degreasing temperature and degreasing time are
set according to the materials of the cell and other factors. The
degreasing temperature can be set to, for example, about
600.degree. C. to 900.degree. C., and the degreasing time can be
set to, for example, about 1 hour to 20 hours.
[0086] 4-5. Formation of Cathode
[0087] The cathode is formed by, for example, forming a layer of
cathode materials according to compacting molding, printing, or a
like process on a laminate of the anode, the electrolyte layer, and
the barrier layer, and then performing calcination. Conditions such
as calcination temperature and calcination time are set according
to the materials of the cell and other factors. The calcination
temperature can be set to, for example, about 900.degree. C. to
1200.degree. C., and the calcination time can be set to, for
example, about 1 hour to 10 hours.
[0088] 4-6. Other Steps
[0089] According to the configuration of the fuel cell, the
production method may include an additional step, or the
above-described steps may be modified. For example, the production
method may include a step of providing a reaction preventive layer
between the electrolyte layer and the cathode, or may include steps
of forming an anode having a two-layer structure (a step of forming
a substrate and a step of forming an anode active layer).
EXAMPLES
[0090] A. Preparation of Cell
[0091] An NiO-8YSZ anode active layer (10 .mu.m), an 8YSZ
electrolyte layer (3 .mu.m), and a GDC barrier layer (3 .mu.m) were
stacked on an NiO-8YSZ anode (500 .mu.m) and calcined together at
1400.degree. C. for 2 hours.
[0092] As shown in Tables 1 to 3, a paste was prepared using
electrode materials (powder) obtained by adding zirconia to
(La.sub.0.6Sr.sub.0.4)(Co.sub.0.2Fe.sub.0.8)O.sub.3,
(La.sub.0.8Sr.sub.0.2)FeO.sub.3, or
La(Ni.sub.0.6Fe.sub.0.4)O.sub.3, and the paste was processed into a
film by screen printing to form a cathode (30 .mu.m) on the barrier
layer. The average particle diameter of the powder measured by a
laser diffraction/scattering particle size distribution analyzer
(LA-700, manufactured by Horiba Ltd.) was 0.5 .mu.m. The cathode
was baked onto the barrier layer by being heated at 1000.degree. C.
for 2 hours.
[0093] An SOFC cell was obtained through the above-described
operation.
[0094] B. Evaluation
[0095] B-1. Power Output Density
[0096] Using the SOFC cell thus prepared, the output density at
0.8V under 750.degree. C. was measured.
[0097] In the case where a cell had a cathode containing
(La.sub.0.6Sr.sub.0.4)(Co.sub.0.2Fe.sub.0.8)O.sub.3 as a principal
component, the cell was evaluated as being good if the cell showed
an output density no smaller than the reference value, 600
mW/cm.sup.2. In Table 1, cells that showed a good output density
are given "good" and otherwise the cells are given "poor".
Likewise, in the case where a cell had a cathode containing
(La.sub.0.8Sr.sub.0.2)FeO.sub.3 as a principal component, the cell
was evaluated as being good if the cell showed an output density of
300 mW/cm.sup.2 or greater, and in the case where a cell had a
cathode containing La(Ni.sub.0.6Fe.sub.0.4)O.sub.3 as a principal
component, the cell was evaluated as being good if the cell showed
an output density of 400 mW/cm.sup.2 or greater.
[0098] B-2. Thermal Cycling Test
[0099] The cells were subjected to a thermal cycling test using an
infrared lamp. A thermal cycling test was performed in which one
cycle consisted of heating to 750.degree. C. in 10 minutes and
cooling to normal temperature in 30 minutes was repeated 100 times,
and the cells were visually inspected with a microscope to look for
peeling at the interface between the cathode and the barrier
layer.
[0100] Cells to which peeling had occurred were evaluated as being
"poor" no matter if the power output density satisfied the
reference value.
[0101] B-3. Observation of Microstructure of Cathode
[0102] The structure of the cathode before and after the thermal
cycling test of section B-2 above was visually inspected with a
scanning electron microscope (SEM).
[0103] C. Results
[0104] The evaluation results of the output density and the thermal
cycling test are presented in Tables 1 to 3.
TABLE-US-00001 TABLE 1 Principal component:
(La.sub.0.6Sr.sub.0.4)(Co.sub.0.2Fe.sub.0.8)O.sub.3 Thermal cycling
test (presence/ Amount of Power absence of zirconia density peeling
after Sample No. (wt. %) (mW/cm.sup.2) 100 cycles) Evaluation 1
1.50 480 No peeling Poor 2 1.00 660 No peeling Good 3 0.650 630 No
peeling Good 4 0.300 680 No peeling Good 5 0.100 650 No peeling
Good 6 0.025 645 No peeling Good 7 0.010 635 No peeling Good 8
0.007 655 No peeling Good 9 0.003 660 No peeling Good 10 0.001 650
Peeled Poor
TABLE-US-00002 TABLE 2 Principal component:
(La.sub.0.8Sr.sub.0.2)FeO.sub.3 Thermal cycling test (presence/
Amount of Power absence of zirconia density peeling after Sample
No. (wt. %) (mW/cm.sup.2) 100 cycles) Evaluation 11 1.50 220 No
peeling Poor 12 1.00 350 No peeling Good 13 0.650 360 No peeling
Good 14 0.300 345 No peeling Good 15 0.100 350 No peeling Good 16
0.025 350 No peeling Good 17 0.010 330 No peeling Good 18 0.007 345
No peeling Good 19 0.003 360 No peeling Good 20 0.001 370 Peeled
Poor
TABLE-US-00003 TABLE 3 Principal component:
La(Ni.sub.0.6Fe.sub.0.4)O.sub.3 Thermal cycling test (presence/
Amount of Power absence of zirconia density peeling after Sample
No. (wt. %) (mW/cm.sup.2) 100 cycles) Evaluation 21 1.50 285 No
peeling Poor 22 1.00 420 No peeling Good 23 0.650 430 No peeling
Good 24 0.300 425 No peeling Good 25 0.100 410 No peeling Good 26
0.025 445 No peeling Good 27 0.010 450 No peeling Good 28 0.007 450
No peeling Good 29 0.003 420 No peeling Good 30 0.001 435 Peeled
Poor
[0105] As shown in Tables 1 to 3, irrespective of the composition
of the principal component, peeling of the cathode was observed
when the additive amount of zirconia was 0.1.times.10.sup.-2 wt %
or less while no peeling was observed when the additive amount of
zirconia was 0.3.times.10.sup.-2 wt % or greater.
[0106] Irrespective of the composition of the principal component,
the resulting output density was low when the additive amount of
zirconia was 1.50 wt % while high output densities were obtained
when the additive amount of zirconia was 1.00 wt % or less.
[0107] The reason for the inhibition of cathode peeling achieved
when the amount of zirconia was 0.3.times.10.sup.-2 wt % or greater
appears to be that due to the addition of zirconia in an
appropriate amount, a solid solution of zirconia and ceria was
formed at the interface between the cathode and the electrolyte
layer, which was in contact with the cathode, and this solid
solution contributed to enhancement of adhesion between the cathode
and the electrolyte layer. Note that although the layer in contact
with the cathode in this example was an electrolyte layer, the same
effect is believed to be demonstrated no matter if the layer in
contact is a different layer such as barrier film.
[0108] In contrast, the output density was lowered when zirconia
was added excessively. Possible reasons therefor may be that the
reaction between zirconia and a cathode component lanthanum (La) or
strontium (Sr) generated lanthanum zirconate, strontium zirconate,
or the like, resulting in: [0109] Increased resistance value of the
cell since the electric conductivity of those reaction products is
low, [0110] Decreased reaction active spots, and/or [0111]
Non-uniform composition of the cathode.
[0112] Moreover, it appears that the addition of zirconia in a
small amount brings about, in addition to the above-described
effects, an effect to stabilize the microstructure of the cathode.
All cathodes had a microstructure as shown in FIG. 2 before the
thermal cycling test. However, after the thermal cycling test, this
structure of the cathode of Sample No.10 was collapsed as shown in
FIG. 4. In contrast, as shown in FIG. 3, the structure of the
cathode of Sample No.4 was maintained even after the thermal
cycling test. Such a structure-maintaining effect was observed in
samples containing zirconia in an amount of 0.03 wt % or greater,
and was prominently observed particularly in samples containing
zirconia in an amount of 0.1 wt % or greater. Although not shown,
the same results were obtained not only when the principal
component of the cathode was
(La.sub.0.6Sr.sub.0.4)(Co.sub.0.2Fe.sub.0.8)O.sub.3 but also
(La.sub.0.8Sr.sub.0.2)FeO.sub.3 and
La(Ni.sub.0.6Fe.sub.0.4)O.sub.3.
[0113] These mechanisms of action of zirconia do not limit the
present invention.
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